Flame4 is rather bad actually, since it is a single texture. The best way is to use SubUVs.
Textures\Clouds8x8Fire.blp or Textures\Clouds8x8ModFire.blp are way better (set a modulate flag for the last one).
Edit, short explanation: SubUV basically means that you use multiple textures in one texture file. That's also where the name comes from, texture coordinates are commonly referred to as U and V and the main texture splits up in multiple rows and columns. The particle appears, its texture starts with the first column/row and then iterates through the columns and rows of the provided texture to show the specific subtextures.
For use of SubUVs, you might wish to take a closer look at building fires, for example:
Environment\LargeBuildingFire\LargeBuildingFire0.mdx